This invention relates to a back-pressure chamber for a scroll compressor wherein seals are positioned to reduce a thrust force between the two scroll members.
Scroll compressors are widely utilized in refrigerant compression applications. Generally, a scroll compressor includes two scroll members each having a base and a generally spiral wrap extending from the base. The wraps interfit to define compression chambers. A motor drives one scroll member to orbit relative to the other. Most scroll compressors have an axially compliant design. In this design, either the orbiting or non-orbiting scroll is allowed to move axially towards the other to minimize leakage between the floor of the scroll base and tips of the opposed scroll wraps by loading one scroll member against the other. This loading prevents the formation of gaps between the floor and tips of the scroll, thus minimizing leakage losses.
However, when the scrolls are loaded against one another, it is possible to damage the scroll components if the created thrust load becomes too high. Further, unwanted frictional losses also increase with increased loading. Thus, it is a goal of a scroll compressor designer to minimize the load as much as possible, while keeping the floor and the tip of the interfitting scroll wrap in contact with each other to avoid leakage.
There are several forces acting on the non-orbiting and orbiting scroll members, shown schematically in FIG. 1. As illustrated for the case of radially compliant orbiting scroll, a projection of a radial gas force vector F.sub.rad passes through a center line of the orbiting scroll 24, and acts at the midpoint of the wrap height. A tangential gas force vector F.sub.tg is perpendicular to the radial gas force and also acts at the midpoint of the wrap height. An axial gas force F.sub.ax is applied normal to the floor or plane of the orbiting scroll. As shown in FIG. 2, to assure that positive contact is maintained between the interfitting scroll members in axial direction, it is necessary to establish a F.sub.BC force which compensates for the force F.sub.ax separating the two scroll members, and also compensates for an overturning moment M.sub.ov which tips the orbiting scroll member relative to the non-orbiting scroll member. The overturning moment M.sub.ov is a product of F.sub.tg and an overturning moment arm L.sub.ov where L.sub.ov extends from the center of orbiting scroll bearing the center of the wraps. The radial gas force F.sub.rad also in theory has an impact on the overturning moment M.sub.ov, but its effect is typically of a second order and is normally neglected in calculations of the overturning moment.
The compensating force F.sub.BC has been provided by tapping a pressurized fluid to a chamber behind one of the two scroll members. Pressurization of this chamber, known as a back-pressure chamber 28 establishes the compensating force F.sub.bc to counteract the separating force F.sub.ax, and the overturning moment M.sub.ov. Typically, back-pressure chambers are defined by at least two seal surfaces, that seal the back-pressure chamber from the suction pressure. The refrigerant is tapped to this chamber through an opening in one of the two scroll members to establish pressure in the back chamber that is higher than suction pressure. Since the pressure in the back chamber exceeds suction pressure, the compensating force F.sub.bc counteracts the separating force F.sub.ax, and the overturning moment M.sub.ov.
As shown in FIG. 3, in prior art designs of back-pressure chambers 28, both an inner seal 12 and an outer seal 10 have been positioned concentrically with respect to the crankcase bore 14. This crankcase seal arrangement while capable of creating a force that counteracts the separating force has the drawback of requiring a high back-pressure chamber force F.sub.bc. As a result, a high thrust load as mentioned above has been encountered between the interfitting scroll members. It is a goal of this invention to reduce the thrust load between the interfitting scroll members.